Heat-assisted magnetic recording (HAMR) medium with optical-coupling multilayer between the recording layer and heat-sink layer

ABSTRACT

A heat-assisted magnetic recording (HAMR) disk has a magnetic recording layer (typically a FePt chemically-ordered alloy), a seed-thermal barrier layer (typically MgO) below the recording layer, a heat-sink layer, and an optical-coupling multilayer of alternating plasmonic and non-plasmonic materials between the heat-sink layer and the seed-thermal barrier layer. Unlike a heat sink layer, the multilayer has very low in-plane and out-of-plane thermal conductivity and thus does not function as a heat sink layer. The multilayer&#39;s low thermal conductivity allows the multilayer to also function as a thermal barrier. Due to the plasmonic materials in the multilayer it provides excellent optical coupling with the near-field transducer (NFT) of the HAMR disk drive.

BACKGROUND Field of the Invention

This invention relates generally to a perpendicular magnetic recordingmedium for use as a heat-assisted magnetic recording (HAMR) medium, andmore particularly to a HAMR medium with improved optical and thermalproperties.

Description of the Related Art

In conventional continuous granular magnetic recording media, themagnetic recording layer is a continuous layer of granular magneticmaterial over the entire surface of the disk. In magnetic recording diskdrives the magnetic material (or media) for the recording layer on thedisk is chosen to have sufficient coercivity such that the magnetizeddata regions that define the data “bits” are written precisely andretain their magnetization state until written over by new data bits. Asthe areal data density (the number of bits that can be recorded on aunit surface area of the disk) increases, the magnetic grains that makeup the data bits can be so small that they can be demagnetized simplyfrom thermal instability or agitation within the magnetized bit (theso-called “superparamagnetic” effect). To avoid thermal instabilities ofthe stored magnetization, media with high magneto-crystalline anisotropy(K_(u)) are required. The thermal stability of a magnetic grain is to alarge extent determined by K_(u)V, where V is the volume of the magneticgrain. Thus, a recording layer with a high K_(u) is important forthermal stability. However, increasing K_(u) also increases thecoercivity of the media, which can exceed the write field capability ofthe write head.

Since it is known that the coercivity of the magnetic material of therecording layer is temperature dependent, one proposed solution to thethermal stability problem is heat-assisted magnetic recording (HAMR),wherein the magnetic recording material is heated locally during writingto lower the coercivity enough for writing to occur, but where thecoercivity/anisotropy is high enough for thermal stability of therecorded bits at the ambient temperature of the disk drive (i.e., thenormal operating temperature range of approximately 15-60° C.). In someproposed HAMR systems, the magnetic recording material is heated to nearor above its Curie temperature. The recorded data is then read back atambient temperature by a conventional magnetoresistive read head.

The most common type of proposed HAMR disk drive uses a laser source andan optical waveguide with a near-field transducer (NFT). A “near-field”transducer refers to “near-field optics”, wherein the passage of lightis through an element with sub-wavelength features and the light iscoupled to a second element, such as a substrate like a magneticrecording medium, located a sub-wavelength distance from the firstelement. The NFT is typically located at the gas-bearing surface (GBS)of the gas-bearing slider that also supports the read/write head andrides or “flies” above the disk surface.

One type of proposed high-K_(u) HAMR media with perpendicular magneticanisotropy is an alloy of FePt (or CoPt) chemically-ordered in the L1₀phase. The chemically-ordered FePt alloy, in its bulk form, is known asa face-centered tetragonal (FCT) L1₀-ordered phase material (also calleda CuAu material). The c-axis of the L1₀ phase is the easy axis ofmagnetization and is oriented perpendicular to the disk substrate. Toobtain the desired chemical ordering to the L1₀ phase, the FePt alloyneeds to be annealed after deposition or deposited with the substratemaintained at high temperatures (e.g., about 500 to 700° C.).

The FePt alloy magnetic layer also typically includes a segregant likeC, SiO₂, TiO₂, TaO_(x), ZrO₂, SiC, SiN, TiC, TiN, B, BC or BN that formsbetween the FePt grains and reduces the grain size. In HAMR media, aseed-thermal barrier layer like MgO is used to induce the desirable(001) texture to the FePt magnetic grains and influence theirgeometrical microstructure and to also act as a thermal barrier layer sothat heat from the NFT is not dissipated too rapidly from the FePtrecording layer. A heat-sink layer is located below the seed-thermalbarrier layer to move heat laterally (in-plane) and then vertically(i.e., in the out-of-plane direction of the recording layer) down to thesubstrate so there will be less heat spreading laterally in therecording layer.

SUMMARY

Heat-sink layers selected from Au, Ag and Cu provide good thermal andoptical properties for HAMR media. The high lateral (in-plane) thermalconductivity of Au, Ag and Cu allows for the heat to be moved laterallyand then down vertically very quickly to the substrate. Also, Au, Ag andCu are plasmonic materials. One definition of a plasmonic material is ametal or metal alloy that has an extinction coefficient k at least twiceas great as the index of refraction n at the wavelength of interest. Assuch, plasmonic materials also provide excellent optical coupling withthe NFT, which results in a confined heat source in the recording layer.

However, the incorporation of a thick Au, Ag, Cu plasmonic layerimmediately below the seed-thermal barrier layer (typically MgO) isdifficult. The recording layer needs to have the right granularstructure and crystallographic orientation to achieve the desiredmagnetic properties. The recording layer is made of FePt L1₀ grains thatare typically separated by thin oxide/nitride segregant materials andrequires a high temperature deposition process. The recording layer alsoneeds to have uniform thickness and be very smooth so the slider can bemaintained just a few nanometers above the disk surface. However, Au, Agand Cu films roughen significantly at high temperatures, and are alsoprone to inter-diffusion when heated. For this reason, an intermediatelayer is required between the Au, Ag or Cu heat-sink layer and theseed-thermal barrier layer. But separating the heat-sink and recordinglayers by too large a distance is detrimental to the thermal and opticalperformance of the medium. For example, the optical benefits ofplasmonic Au, Ag, Cu disappear when used under a 10 to 25 nm thickintermediate layer.

In embodiments of this invention, an optical-coupling multilayer ofalternating plasmonic and non-plasmonic materials is located between theseed-thermal barrier layer and the heat-sink layer without the need foran intermediate layer. Alternatively, the multilayer may be locatedwithin the seed-thermal barrier layer. Unlike a heat sink layer, themultilayer has very low in-plane and out-of-plane thermal conductivityand thus does not function as a heat sink layer. For that reason, aseparate layer of heat-sink material is required below the multilayer.The multilayer's low thermal conductivity allows the multilayer to alsofunction as a thermal barrier. Due to the plasmonic materials in themultilayer it provides excellent optical coupling with the NFT. Becauseof the lamination the multilayer provides good stability upon annealing.

It is important that the HAMR medium has a high thermal gradient (TG) inthe recording layer, meaning there is a sharp drop in temperature at theedges of the bits being recorded. Similarly, the required laser power(LP) to achieve an acceptable thermal gradient, which is largelydetermined by the optical and thermal properties of the layers below therecording layer, should be minimized to prolong the life of the NFT. Theoptical coupling multilayer in embodiments of this invention improvesthe ratio of TG/LP over HAMR media without a single plasmonic layer.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a top view of a heat-assisted magnetic recording (HAMR) diskdrive according to the prior art.

FIG. 2 depicts a sectional view, not drawn to scale because of thedifficulty in showing the very small features, of a gas-bearing sliderfor use in HAMR disk drive and a portion of a HAMR disk according to theprior art.

FIG. 3 is a sectional view showing a HAMR disk with a single plasmonicheat-sink layer of Au, Ag or Cu according to the prior art.

FIG. 4 is a sectional view showing a HAMR disk according to anembodiment of the invention.

FIG. 5 is a sectional view showing a HAMR disk according to anotherembodiment of the invention.

FIG. 6 is a table of the measured in-plane thermal conductivity(TC_(IP)) for RuAl/Au and RuAl/Rh multilayers of various thicknesses andnumber of repeats of the alternating layers formed on a glass substratewith a thin 2 nm Au or Rh cap to prevent oxidation of the multilayer.

FIG. 7 shows graphical data for thermal gradient/laser power (TG/LP)from computer modeling for RuAl (1 nm)/Au(1 nm) multilayers of variousthicknesses compared to a single Au plasmonic layer of variousthicknesses.

FIG. 8 shows graphical data for TG/LP from computer modeling for RuAl(1nm)/Rh(1 nm) multilayers of various thicknesses compared to a single Auplasmonic layer of various thicknesses.

DETAILED DESCRIPTION

FIG. 1 is a top view of a heat-assisted magnetic recording (HAMR) diskdrive 100 according to the prior art. In FIG. 1, the HAMR disk drive 100is depicted with a disk 200 with a continuous magnetic recording layer31 with concentric circular data tracks 118. Only a portion of a fewrepresentative tracks 118 near the inner and outer diameters of disk 200are shown.

The drive 100 has a housing or base 112 that supports an actuator 130and a drive motor for rotating the magnetic recording disk 200. Theactuator 130 may be a voice coil motor (VCM) rotary actuator that has arigid arm 131 and rotates about pivot 132 as shown by arrow 133. Ahead-suspension assembly includes a suspension 135 that has one endattached to the end of actuator arm 131 and a head carrier, such as agas-bearing slider 120, attached to the other end of suspension 135. Thesuspension 135 permits the slider 120 to be maintained very close to thesurface of disk 200 and enables it to “pitch” and “roll” on the bearingof gas (typically air or helium) generated by the disk 200 as it rotatesin the direction of arrow 20. The slider 120 supports the HAMR head (notshown), which includes a magnetoresistive read head, an inductive writehead, the near-field transducer (NFT) and optical waveguide. Asemiconductor laser 90, for example with a wavelength of 780 to 980 nm,may be used as the HAMR light source and is depicted as being supportedon the top of slider 120. Alternatively, the laser may be located onsuspension 135 and coupled to slider 120 by an optical channel. As thedisk 200 rotates in the direction of arrow 20, the movement of actuator130 allows the HAMR head on the slider 120 to access different datatracks 118 on disk 200. The slider 120 is typically formed of acomposite material, such as a composite of alumina/titanium-carbide(Al₂O₃/TiC). Only one disk surface with associated slider and read/writehead is shown in FIG. 1, but there are typically multiple disks stackedon a hub that is rotated by a spindle motor, with a separate slider andHAMR head associated with each surface of each disk.

In the following drawings, the X direction denotes a directionperpendicular to the gas-bearing surface (GBS) of the slider, the Ydirection denotes a track width or cross-track direction, and the Zdirection denotes an along-the-track direction. FIG. 2 is a schematiccross-sectional view illustrating an example of a HAMR head according tothe prior art, which is also capable of functioning as the HAMR head inembodiments of this invention. In FIG. 2, the disk 200 is depicted as aconventional disk with the HAMR recording layer 31 being a continuousnon-patterned magnetic recording layer of magnetizable material withmagnetized regions or “bits” 34. The bits 34 are physically adjacent toone another and the boundaries of adjacent bits are referred to asmagnetic transitions 37. The bits are recorded in individual datasectors. The recording layer 31 is typically formed of a high-anisotropy(K_(u)) substantially chemically-ordered FePt alloy (or CoPt alloy) withperpendicular magnetic anisotropy. The disk includes an overcoat 36,typically formed of amorphous diamond-like carbon (DLC), and a liquidlubricant layer 38, typically a bonded perfluropolyether (PFPE).

The gas-bearing slider 120 is supported by suspension 135. The slider120 has a recording-layer-facing surface 122 onto which an overcoat 124is deposited. The overcoat 124 is typically a DLC overcoat with athickness in the range of about 10 to 30 Å and whose outer surface formsthe GBS of the slider 120. An optional adhesion film or undercoat (notshown), such as a 1 to 5 Å silicon nitride (SiN_(x)) film, may bedeposited on the surface 122 before deposition of the overcoat 124. Theslider 120 supports the magnetic write head 50, magnetoresistive (MR)read head 60, and magnetically permeable read head shields S1 and S2. Arecording magnetic field is generated by the write head 50 made up of acoil 56, a main magnetic pole 53 for transmitting flux generated by thecoil 56, a write pole 55 with end 52, and a return pole 54. A magneticfield generated by the coil 56 is transmitted through the magnetic pole53 to the write pole end 52 located near an optical near-fieldtransducer (NFT) 74. The write head 50 is typically capable of operatingat different clock rates so as to be able to write data at differentfrequencies. The NFT 74, also known as a plasmonic antenna, typicallyuses a low-loss metal (e.g., Au, Ag, Al or Cu) shaped in such a way toconcentrate surface charge motion at a tip located at the slider GBSwhen light from the waveguide 73 is incident. Oscillating tip chargecreates an intense near-field pattern, heating the recording layer 31.The metal structure of the NFT 74 can create resonant charge motion(surface plasmons) to further increase intensity and heating of therecording layer 31. At the moment of recording, the recording layer 31of disk 200 is heated by the optical near-field generated by the NFT 74and, at the same time, a region or “bit” 34 is magnetized and thuswritten onto the recording layer 31 by applying a recording magneticfield generated by the write pole end 52.

A semiconductor laser 90 is mounted to the top surface of slider 120. Anoptical waveguide 73 for guiding light from laser 90 to the NFT 74 isformed inside the slider 120. The laser 90 is typically capable ofoperating at different power levels. Materials that ensure a refractiveindex of the waveguide 73 core material to be greater than a refractiveindex of the cladding material may be used for the waveguide 73. Forexample, Al₂O₃ may be used as the cladding material and TiO₂, Ta₂O₅ andSiO_(X)N_(y) as the core material. Alternatively, SiO₂ may be used asthe cladding material and Ta₂O₅, TiO₂, SiO_(X)N_(y), or Ge-doped SiO₂ asthe core material. The waveguide 73 that delivers light to NFT 74 ispreferably a single-mode waveguide.

FIG. 3 is a sectional view showing HAMR disk 200 with a continuousgranular recording layer (RL) 31 according to the prior art. Therecording layer 31 may be comprised of a substantiallychemically-ordered FePt alloy (or CoPt alloy) with or without segregantsas proposed in the prior art. The disk 200 is a substrate 201 having agenerally planar surface on which the representative layers aresequentially deposited, typically by sputtering. The hard disk substrate201 may be any commercially available high-temperature glass substrate,but may also be an alternative substrate, such as silicon orsilicon-carbide. An adhesion layer 202, typically about 10-200 nm of anamorphous adhesion layer material like a CrTa or NiTa alloy, isdeposited on substrate 201.

An optional soft underlayer (SUL) 204 of magnetically permeable materialthat serves as a flux return path for the magnetic flux from the writehead may be formed on the adhesion layer 202. The SUL 204 may be formedof magnetically permeable materials that are also compatible with thehigh-temperature deposition process for FePt, such as certain alloys ofCoFeZr and CoZr. The SUL 204 may also be a laminated or multilayered SULformed of multiple soft magnetic films separated by nonmagnetic films,such as electrically conductive films of Al or CoCr. The SUL 204 mayalso be a laminated or multilayered SUL formed of multiple soft magneticfilms separated by interlayer films that mediate an antiferromagneticcoupling, such as Ru, Ir, or Cr or alloys thereof. The SUL 204 may havea thickness in the range of about 5 to 100 nm.

A seed layer 205, for example a layer of RuAl or NiAl, is deposited onSUL 204, or on adhesion layer 202 if no SUL is used. A heat-sink layer206 is then deposited on seed layer 205. The heat-sink layer 206facilitates the transfer of heat away from the RL to prevent spreadingof heat to regions of the RL adjacent to where data is desired to bewritten, thus preventing overwriting of data in adjacent data tracks.The heat-sink layer 206 may be formed of plasmonic materials Au, Ag orCu, which have high thermal conductivity and allow excellent couplingwith the NFT, which results in a confined heat source. However, Au, Agand Cu roughen significantly when annealed at high temperature. For thisreason, the seed-thermal barrier layer 210 for the RL cannot be formeddirectly on the heat-sink layer 206. Thus, an intermediate layer (IL)207 is required between the Au, Ag or Cu heat-sink layer 206 and theseed-thermal barrier layer 210. The seed-thermal barrier layer 210 isformed on the IL 207 and acts as both the seed layer for the RL 31 and athermal barrier layer. The seed-thermal barrier layer 210 is typicallyMgO, but other materials have been proposed, including CrRu, CrMo, TiNand a mixture of MgO and TiO₂ (MTO) like (Mg_(0.2)Ti_(0.8))O. However,the IL 207 increases the distance between the RL 31 and the heat-sinklayer 206, which reduces the optical and thermal performance of theheat-sink layer 206. U.S. Pat. No. 8,605,555 B1, which is assigned tothe same assignee as this application, describes a HAMR medium with anamorphous IL like CrTi, CrTa or NiTa, between the heat-sink layer andthe FePt RL to reduce the roughness caused by the heat-sink layer. U.S.Pat. No. 9,558,777 B2, which is assigned to the same assignee as thisapplication, describes a HAMR medium with a heat-sink layer that may beformed from a long list of metals and alloys, including plasmonic Au,Ag, Cu and Rh, but requires an IL like amorphous NiTa between theheat-sink layer and the MgO seed layer. Heat-sink layers selected fromnon-plasmonic materials Cr, W, Mo and Ru have been proposed in place ofAu, Ag or Cu because they do not roughen when annealed and thus may notrequire an intermediate layer. However, these materials provide lessthan optimal optical and thermal properties.

The perpendicular media that forms the RL 31 is a high-anisotropy(K_(u)) substantially chemically-ordered FePt alloy (or CoPt alloy) withperpendicular magnetic anisotropy. Substantially chemically-orderedmeans that the FePt alloy has a composition of the formFe_((y))Pt_((100-y)) where y is between about 45 and 55 atomic percent.Such alloys of FePt (and CoPt) ordered in L1₀ are known for their highmagneto-crystalline anisotropy and magnetization, properties that aredesirable for high-density magnetic recording materials. Thesubstantially chemically-ordered FePt alloy, in its bulk form, is knownas a face-centered tetragonal (FCT) L1₀-ordered phase material (alsocalled a CuAu material). The c-axis of the L1₀ phase is the easy axis ofmagnetization and is oriented perpendicular to the disk substrate. Thesubstantially chemically-ordered FePt alloy may also be a pseudo-binaryalloy based on the FePt L1₀ phase, e.g., (Fe_((y))Pt_((100-y)))—X, wherey is between about 45 and 55 atomic percent and the element X may be oneor more of Ni, Au, Cu, Pd, Mn and Ag and present in the range of betweenabout 0% to about 20% atomic percent. While the pseudo-binary alloy ingeneral has similarly high anisotropy as the binary alloy FePt, itallows additional control over the magnetic and other properties of theRL. For example, Ag improves the formation of the L1₀ phase and Cureduces the Curie temperature. While the HAMR media according toembodiments of the invention will be described with a FePt RL,embodiments of the invention are also fully applicable to media with aCoPt (or a pseudo-binary CoPt—X alloy based on the CoPt L1₀ phase) RL.

FePt L1₀ phase based granular thin films exhibit strong perpendicularanisotropy, which potentially leads to small thermally stable grains forultrahigh density magnetic recording. To fabricate small grain FePt L1₀media some form of segregant to separate grains can be used as anintegral part of the magnetic recording layer. Thus in the HAMR media,the RL 31 also typically includes a segregant, such as one or more of C,SiO₂, TiO₂, TaO_(x), ZrO₂, SiC, SiN, TiC, TiN, B, BC, and BN that formsbetween the FePt grains and reduces the grain size. While FIG. 3 depictsthe RL 31 as a single magnetic layer, the recording layer may be amultilayer, for example multiple stacked FePt sublayers, each with adifferent segregant, as described in U.S. Pat. No. 9,406,329 B1, whichis assigned to the same assignee as this application.

The FePt RL is sputter deposited, typically to a thickness of betweenabout 4 to 15 nm, while the disk substrate 201 is maintained at anelevated temperature, for example between about 500 and 700° C. The FePtRL may be sputter deposited from a single composite target havinggenerally equal atomic amounts of Fe and Pt and with the desired amountsof X-additives and segregant, or co-sputtered from separate targets.

An optional capping layer 212, such as a thin film of Co, may be formedon the RL 31. A protective overcoat (OC) 36 is deposited on the RL 31(or on the optional capping layer 212), typically to a thickness betweenabout 1-5 nm. OC 36 is preferably a layer of amorphous diamond-likecarbon (DLC). The DLC may also be hydrogenated and/or nitrogenated, asis well-known in the art. On the completed disk, a liquid lubricant 38,like a perfluorpolyether (PFPE), is coated on OC 36.

FIG. 4 is a sectional view of a HAMR disk according to an embodiment ofthe invention showing the optical coupling multilayer 300 between theseed-thermal barrier layer 210 and the heat-sink layer 206. In FIG. 4the optional SUL layer is omitted. The seed-thermal barrier layer 210 ispreferably MgO or MTO. The multilayer 300 comprises alternating layersof a non-plasmonic material 302 and plasmonic material 304. Each layer302, 304 has a thickness in the range of 0.5-2 nm and the totalthickness of the multilayer 300 is preferably in the range of 3-20 nm.

FIG. 5 is a sectional view of a HAMR disk according to anotherembodiment of the invention showing the optical coupling multilayer 300between a first seed-thermal barrier film 220 and a second seed-thermalbarrier film 230. Each of films 220, 230 may be formed of MgO or MTO.

Table 1 below lists various metals and metal alloys that can be used forlayers 302 and 304 with their corresponding n and k values at awavelength of 830 nm. In addition, various metal nitrides like CrN, VN,WN, MoN may be suitable as non-plasmonic materials because they have alattice constant like Au and Ag and exhibit low bulk thermalconductivity.

TABLE1 n k Plasmonic Au 0.1 5.3 Ag 0.1 5.0 Cu 0.3 5.3 Rh 2.8 7.0Non-plasmonic Ru₅₀Al₅₀ 4.3 4.4 Ni₅₀Ta₅₀ 3.9 4.0 Cr₅₀Ta₅₀ 4.3 4.4

The multilayer 300 is made of alternating thin plasmonic layersseparated by thin non-plasmonic material. The thickness of eachindividual layer is small relative to each material's electron mean freepath, which significantly lowers its thermal conductivity. As a result,the multilayer 300 has low in-plane thermal conductivity (TC_(IP)),preferably less than about 20 W/mK, and thus does not function as aheat-sink layer. For this reason, heat-sink layer 206 is required belowmultilayer 300 and may be formed of any of the known heat-sink materialsincluding Cr, W, Mo, Ru, Rh, Au, Ag, or Cu and their alloys. However,Cr, W and Mo and their alloys are preferred because they do not roughenwhen annealed. FIG. 6 is a table that lists the measured TC_(IP) forRuAl/Au and RuAl/Rh multilayers of various thicknesses and number ofrepeats of the alternating layers formed on a glass substrate with athin 2 nm Au or Rh cap to prevent oxidation of the multilayer. TheRuAl/Au multilayers exhibit TC_(IP) around 20 W/mK. The RuAl/Rhmultilayers exhibit TC_(IP) around 10 W/mK. The multilayer 300 also hasanisotropic thermal conductivities, i.e., the out-of-plane thermalconductivity (TC_(OP)) is lower than the TC_(IP). This arises as heatcarriers, electrons and/or phonons, encounter a lot of interfaces in theout-of-plane direction, which results in increased scattering andreduced conductivity. The interface thermal conductance between metalsis typically in the range of 500 MW/m²K to 4000 MW/m²K depending on thematerials and the quality of the interfaces. This results in multilayershaving a TC_(OP) in the range of 0.5 to 10 W/mK, depending on eachinterface thermal conductance and layer thicknesses. Based on TC_(IP)measurements, TC_(OP) is estimated around 10 W/mK for the RuAl/Aumultilayers and between 5 and 10 W/mK for the RuAl/Rh multilayers. Bycomparison a conventional heat-sink material like Cr has a TC_(IP)around 40-45 W/mK and a TC_(OP) around 40-45 W/mK.

The optical performance of the HAMR medium stack can be modeled by theratio of thermal gradient TG (change in temperature in thealong-the-track direction) over the required laser power (LP) to write a48-nm-wide track. The higher the ratio the better the optical efficiencyof the medium. FIG. 7 shows graphical TG/LP data from computer modelingfor RuAl(1 nm)/Au(1 nm) multilayers of various thicknesses withTC_(IP)=20 W/mK and TC_(OP)=10 W/mK compared to a single Au plasmoniclayer of various thicknesses. The reference (TG/LP=1) is for a stackwith no plasmonic layer below the seed-thermal barrier layer. The 3 nmthick multilayer is RuAl(1 nm)/Au(1 nm)/RuAl(1 nm) with the RuAldirectly on the heat-sink layer and directly below the seed-thermalbarrier layer. The 9 nm thick multilayer is 4 repeats of the 3 nm thickmultilayer. FIG. 8 shows the same graphical TG/LP data from computermodeling as FIG. 7, but for RuAl(1 nm)/Rh(1 nm) multilayers for twocases, one where TC_(IP)=10 W/mK and TC_(OP)=5 W/mK and one whereTC_(IP)=10 W/mK TC_(OP)=10 W/mK.

The modeled data for both FIGS. 7 and 8 show the optical couplingprovided by the multilayer, namely an improvement in TG/LP over thereference, with the improvement increasing with multilayer thickness(increased number of repeat laminations). Similar improvements in TG/LPhave also been shown for modeled data for RuAl(2 nm)/Au(2 nm) and RuAl(2nm)/Au(2 nm) multilayers of various thicknesses. As shown by FIGS. 7 and8, TG/LP increases with increasing multilayer thickness, with thepreferred range of thickness being between about 3-20 nm.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

What is claimed is:
 1. A heat-assisted magnetic recording medium comprising: a substrate; a heat-sink layer on the substrate; a magnetic recording layer comprising a chemically-ordered alloy selected from a FePt alloy and a CoPt alloy; a seed-thermal barrier layer for the recording layer, wherein the recording layer is on and in contact with the seed-thermal barrier layer; and a multilayer comprising alternating layers of a plasmonic material and a non-plasmonic material, the multilayer being between the heat-sink layer and the seed-thermal barrier layer.
 2. The medium of claim 1 wherein the plasmonic material is selected from Au, Ag, Cu and Rh.
 3. The medium of claim 1 wherein the non-plasmonic material is selected from a RuAl alloy, a NiTa alloy, a CrTa alloy and a nitride of Cr, V, W or Mo.
 4. The medium of claim 1 wherein each of the layers of plasmonic and non-plasmonic material has a thickness greater than or equal to 0.5 nm and less than or equal to 2 nm.
 5. The medium of claim 1 wherein the multilayer has a thickness greater than or equal to 3 nm and less than or equal to 20 nm.
 6. The medium of claim 1 wherein the heat-sink layer is formed of material selected from Cr, W, Mo and their alloys.
 7. The medium of claim 1 wherein the seed-thermal barrier layer is selected from MgO and MTO.
 8. The medium of claim 1 wherein a layer of non-plasmonic material in the multilayer is on and in contact with the heat-sink layer.
 9. The medium of claim 1 wherein the magnetic recording layer further comprises a substantially chemically-ordered alloy comprising Pt and an element selected from Fe and Co, and a segregant selected from one or more of C, SiO₂, TiO₂, TaO_(x), ZrO₂, SiC, SiN, TiC, TiN, B, BC and BN.
 10. The medium of claim 1 wherein the multilayer is on and in contact with the heat sink layer and the seed-thermal barrier layer is on and in contact with the multilayer.
 11. The medium of claim 1 wherein the seed-thermal barrier layer comprises first and second films, wherein the multilayer is between the first and second films, the first film is on and in contact with the heat-sink layer, the second film is on and in contact with the multilayer, and the recording layer is on and in contact with the second film.
 12. The medium of claim 11 wherein each of the first and second seed-thermal barrier layer films is selected from MgO and MTO.
 13. The medium of claim 11 wherein the plasmonic material is selected from Au, Ag, Cu and Rh, and the non-plasmonic material is selected from a RuAl alloy, a NiTa alloy, a CrTa alloy and a nitride of Cr, V, W or Mo.
 14. A heat assisted magnetic recording (HAMR) disk drive comprising: the medium according to claim 1 wherein said medium is a rotatable HAMR disk; and a carrier maintained near the magnetic recording layer of the disk and supporting a near-field transducer.
 15. A heat-assisted magnetic recording (HAMR) disk comprising: a disk substrate; a heat-sink layer on the substrate; a multilayer comprising alternating layers of a plasmonic material and a non-plasmonic material on the heat-sink layer, the plasmonic material being selected from Au, Ag, Cu and Rh, and the non-plasmonic material being selected from a RuAl alloy, a NiTa alloy, a CrTa alloy and a nitride of Cr, V, W or Mo; a seed-thermal barrier layer selected from MgO and MTO on and in contact with the multilayer; and a magnetic recording layer comprising a chemically-ordered alloy selected from a FePt alloy and a CoPt alloy on and in contact with the seed-thermal barrier layer.
 16. The disk of claim 15 wherein each of the layers of plasmonic and non-plasmonic material has a thickness greater than or equal to 0.5 nm and less than or equal to 2 nm and wherein the multilayer has a thickness greater than or equal to 3 nm and less than or equal to 20 nm.
 17. A heat assisted magnetic recording (HAMR) disk drive comprising: the disk according to claim 15; and a gas-bearing slider maintained near the magnetic recording layer of the disk and supporting a near-field transducer.
 18. A heat-assisted magnetic recording (HAMR) disk comprising: a disk substrate; a heat-sink layer on the substrate; a first seed-thermal barrier film selected from MgO and MTO on the heat-sink layer; a multilayer comprising alternating layers of a plasmonic material and a non-plasmonic material on the first seed-thermal film, the plasmonic material being selected from Au, Ag, Cu and Rh, and the non-plasmonic material being selected from a RuAl alloy, a NiTa alloy, a CrTa alloy and a nitride of Cr, V, W or Mo; a second seed-thermal barrier film selected from MgO and MTO on and in contact with the multilayer; and a magnetic recording layer comprising a chemically-ordered alloy selected from a FePt alloy and a CoPt alloy on and in contact with the second seed-thermal barrier film.
 19. The disk of claim 18 wherein each of the layers of plasmonic and non-plasmonic material has a thickness greater than or equal to 0.5 nm and less than or equal to 2 nm and wherein the multilayer has a thickness greater than or equal to 3 nm and less than or equal to 15 nm.
 20. A heat assisted magnetic recording (HAMR) disk drive comprising: the disk according to claim 18; and a gas-bearing slider maintained near the magnetic recording layer of the disk and supporting a near-field transducer. 